1. Field of the Invention
The present invention involves the field of radio communications. More particularly, the present invention involves a radio transmitter that compensates for the effects of aging and temperature on the transistors within the radio transmitter.
2. Discussion of the Related Art
Radio Frequency (RF) communication is becoming more ubiquitous with the proliferation of miniaturized electronics and more sophisticated digital signal processing techniques. More RF communications subsystems are being deployed in environments, such as aircraft, whereby more information may be communicated and processed in an automated fashion. In the case of aircraft, communication schemes such as VDL (VHF Data Link) provide automated communication of aircraft status and navigation information to air traffic controllers without encumbering the air crew. Such communication schemes enable global air traffic management systems to have timely information regarding the location and status of aircraft.
Communication schemes such as VDL require additional RF communications links between the aircraft and the air traffic control system. Accordingly, aircraft employing VDL require additional RF transmitters.
Integrating additional RF transmitters into an aircraft requires consideration of available mass, space, and power. Further, there is considerable motivation to minimize the cost of additional RF transmitters.
Given the high data rates of modern communication schemes such as VDL, rigorous requirements are placed on an RF transmitter. An RF transmitter must perform consistently with regard to output power, gain, linearity, inter-stage matching, and adjacent channel power (hereinafter “performance characteristics”). Unfortunately, the transistors within an RF transmitter are susceptible to changes in performance characteristics, depending on their temperature and age. Age-induced changes in performance characteristics generally result from increased gate leakage and changes in drain-source resistance, which are primarily due to the gradual introduction of defects in the crystalline structure of the drain-source channel.
It is possible to compensate for changes in the transistor to maintain its performance characteristics. In the case of Field Effect Transistors (FETs), changes in performance characteristics may be compensated by adjusting the bias voltage across the gate and the source (Vgs) of the FET. Similar bias adjustments may be made to compensate for changes in performance characteristics of other types of transistors.
Related art solutions for maintaining performance characteristics of the transistors include the use of RF feedback in a closed loop control implementation. In RF feedback, an RF transmitter monitors its own RF output power. An embedded processor or analog circuit compares the expected RF output with the measured RF output and adjusts the transmitter accordingly.
Problems with the related art solution include the following. First, additional hardware is needed to monitor RF output power, which increases the mass, space, power consumption, and cost of the RF transmitter. Factors such as space, mass and power consumption are critical in aircraft-based applications. Second, for applications involving complex modulation, the real-time closed loop control implementation is generally computationally intensive and either requires considerable processor resources, or requires complex analog circuitry.
Accordingly, there is a need for an RF transmitter, which compensates for the effects of aging and temperature on the performance characteristics of its transistors, while substantially minimizing size, mass, power consumption, cost, and complexity.